Procedure and apparatus for nitriding film type precision resistors



March 15, 1966 G. A. SWARTZ ETAL 3,240,637

PROCEDURE AND APPARATUS FOR NITRIDING FILM TYPE PRECISION RESISTORS 2Sheets-Sheet 1 Filed Feb. 19, 1962 3! 2 M f COAO'IkING mmvroxs GAYLORDA. SWARTZ WILLIAM E. MCLEAN b wogiflfiw lflfiim March 1966 G. A. SWARTZETAL 3,240,537

PROCEDURE AND APPARATUS FOR NITRIDING FILM TYPE PRECISION RESISTORSFiled Feb. 19, 1962 2 Sheets-Sheet 2 TEMP CONTROL FOUR WAY ENOID VALVESO L E NOI D REVERSNG TIMER LL INVENTORS CAVLORD A. SWARTZ WlLLlAM E. McLEAN h m (911, 7W, z/mxwm 3,240,637 PROCEDURE AND APPARATUS FORNITRIDING FILM TYPE PRECISION RESISTORS Gaylord A. Swartz and William E.McLean, Independence, Kans., assignors to Electra Manufacturing Company,Kansas City, Mo., a corporation of Missouri Filed Feb. 19, 1962, Ser.No. 173,975 3 Claims. (Cl. 14816.6)

The present invention relates to manufacture of film type precisionresistors and more particularly to means for nitriding films ofchromium-titanium alloys or the like.

It is an object to provide a procedure and apparatus for nitridingalloyed films to produce a conductive coating which is stable, adherentand durable and to produce film type resistors capable of maintainingtheir physical and electrical qualities over long periods of time andeven though subjected to severe operating conditions. It. is anotherobject related to the foregoing to provide a procedure and apparatus fornitridingfilms made of metallic alloys, for example, chromium andtitanium, and which produces a substantially flat temperaturecoefficient of resistance over wide temperature swings. Consequently, itis an object to provide nitrided resistors capable of meeting the moststringent military and commercial specificatrons.

It is another object to provide a nitriding procedure and apparatuswhich is capable of handling large batches of individual elements, forexample, resistor substrates, but which nevertheless assures uniformnitriding over each unit area of the elements being treated. It is arelated object to provide a nitriding procedure and apparatus whichpermits treatment of the elements in bulk while still insuring a highdegree of uniformity from unit to unit and which nevertheless provides arelatively short treatment cycle. Consequently it is an object toprovide a procedure and apparatus for making nitrided film typeresistors at low cost, a cost comparable to the cost of manufacturingconventional non-precision resistors.

Other objects of the present invention will become apparent upon readingthe attached detailed description and upon reference to the drawings inwhich:

FIGURE 1 is a general perspective view of a portion of the nitridingapparatus constructed in accordance with the present invention.

FIG. 2 is a longitudinal section taken vertically through one end of thenitriding tube with a charge of resistor substrates in place.

FIG. 2a is a cross section taken along line 2a.2a in FIG. 2.

FIG. 3 is a diagram showing the means for supplying ammonia gas to thenitriding tube and the control system therefor.

While the invention has been described in connection with a preferredembodiment, it will be understood that we do not intend to be limited tothe embodiment described, but intend to cover the various modified andalternative procedures and apparatus included within the spirit andscope of the appended claims.

Turning to the drawings, the preferred apparatus for practicing theinvention includes a nitriding chamber generally indicated at havingconnections 11, 12 at its ends. Preferably, the chamber is tubular andis of horizontally disposed U shape to facilitate movement into and outof a high temperature oven 15 having an opening 16. For the purpose ofsupporting the nitriding tube, a wheeled carriage 17 is provided havingwheels 18 for riding on a pair of tracks 19. For sealing the opening 16when the nitriding tube is inserted, the tube has a perpendicularlyextending sealing plate 20. The plate may be made of metal having arefractory surface 21 of sheet asbestos or the like. A material ischosen for the tube 10 which is non-porous, refractory and inert andwhich has i i d States Patent 0 ice a low temperature coefiicient ofexpansion, fused quartz being preferred.

To permit loading of the nitriding chamber with resistor elements havinga conductive alloy film thereon, the ends 11, 12 of the tube arepreferably constructed as shown in FIG. 2. The end 11 will be taken asrepresentative. In this figure, it will be noted that the refractoryquartz portion of the tube terminates in a metallic sleeve 31 of brassor the like having a removable end cap 32 provided with a circulargasket 33 of heat resistant rubber. For maintaining the end cap firmlyseated, a clamping yoke 34 is provided which is swingable to anout-of-the-way position upon a pair of pivot bolts 35, 36 tapped intothe sleeve 31. Threaded into the yoke are clamping screws 37 which bearupon the outer surface of the end caps. Gas connections 38, 39 areprovided in the cap 32, the connection 38 serving to conduct gas intothe nitriding chamber and, in another portion of the cycle, to exhaustgas therefrom. Corresponding reference numerals, with subscript a, applyto corresponding parts at the opposite end of the tube.

The sleeves 31, 31a and their junctions with the tube 16 are kept coolnothwithstanding the high temperature within the oven by an air jacket40 which lies next to the plate 20 and which surrounds the ends of thetube as shown. Inlet and outlet connections 41, 42 are provided and airflow is induced by any desired means.

To load the nitriding chamber with a charge of resistor elements, theelements indicated at 45 are stacked in a semi-circular tray 46 made ofstainless steel or the like and which conforms to the curvature of theinner wall of the tube. It is convenient to stack the elements end toend in horizontal rows since the units are received from the vacuumchamber strung on lengths of wire. Moreover, this mode of stackinginsures maximum utilization of the space within the tube. Similar trays46 are placed in both legs of the U. While uniform stacking ispreferred, it will be apparent that the invention is not limited theretoandthat the resistor elements may be stacked randomly in the chamber.After the nitriding procedure, to be described in some detail below, hasbeen completed the nitriding tube 10 is removed from the oven byretracting carriage 17 along the tracks 19. The clamping screws 37, 37aare released so that the yoke 34, 34a may be swung to one side. The caps32, 32a are removed so that the trays of resistor elements may bewithdrawn.

In accordance with the present invention means are provided forthrottling ammonia gas from a source which is substantially aboveatmospheric pressure to a pressure which is substantially below that ofthe atmosphere for flowing through the nitriding chamber while thechamber is maintained at a high temperature in the oven 15. The ammoniagas used in the nitriding chamber is derived from an ammonia cylinder 50which may have a pressure, at room temperature, on the order of poundsper square inch. In the present instance throttling of gas to thenitriding chamber takes place in two steps. Thus throttling takes placefirst in the needle valve 51 and subsequently in a second needle valve52. Use of two valves in this position is accompanied by severaladvantages. In the first place the range of pressure from aproximately130 pounds at the input to less than atmospheric at the output is sogreat that reliability is improved by throttling in two steps, from highpresssure to intermediate pressure and from intermediate to lowpressure. A further advantage resides in the fact that a flow meter 53may be interposed between the two valves and, by relative adjustment ofthem, a pressure may be caused to exist at the flow meter which achievesmaximum accuracy in the measurement of flow. Finally, use of two valvesmakes the throttling adjustment less critical than if all of thethrottling had to be accomplished with a single valve.

Where most of the t-hrottling effect is achieved with one of the valves,the other may be used to provide Vernier adjustment.

In accordance with one of the aspects of the invention a vacuum pump isemployed for exhausting gas from the nitriding chamber and a valvecapable of throttling action is interposed between the chamber and thepump for providing additional control of the pressure in the chamber. Inthe present instance the vacuum pump, indicated at 55, has an associatedvalve 56 which may be a needle valve or valve of other design capable ofthrottling action. By proper relative adjustment of valves 51 and 52 onthe one hand and valve 56 on the other, it will be apparent that almostany desired pressure below atmospheric may be established in thenitriding chamber for a given rate of flow. Moreover, by relativeadjustment of all three of the Valves the flow measuring device 53 andthe nitriding chamber may both be operated at optimum pressure undergiven rate of flow conditions providing a high degree of flexibility ofadjustment.

It may be noted that it is not essential that the valve 56 be of thethrottling type provided that the line which leads from the nitridingchamber to the vacuum pump is tailored both in length and insidediameter so as to produce the desired amonut of throttling action, inwhich case the valve 56 may be replaced by a simple valve of the on-otftype.

In order to control the direction of gas flow through the nitridingtube, a four-way solenoid type valve 60 is interposed between thenitriding tube and the remainder of the system. Such valve 'has an inletport 61, an outlet port 62 and switchable ports 63, 64. The valve 60 isof conventional type, capable of making either straight through or crossconnections. Assuming a straightthrough connection, gas at low pressureis fed via the needle valves 51, 52 into the first end 11 of thenitriding tube while gas is withdrawn from the second end 12 throughvalve 56 and vacuum pump 55.

To enable reversal of the direction of fiow on a periodic or timedbasis, a solenoid reversing timer 65 is employed to control theenergization of the winding in the solenoid valve 60. Thus, in the caseof a nitriding treatment of, say, one hours duration, the ammonia gasmay be caused to flow through the U tube in one direction for one halfhour and in the reverse direction for the remaining half hour, therebytending to equalize the treatment of all of the resistor elements in thetube. Or, if desired, the timer 65 may be so constructed as to energizethe solenoid valve for shorter intervals, for example, so that flowtakes place in one direction for a period of fifteen minutes and in theopposite direction for a period of fifteen minutes with two completecycles in a period of one hour.

The degree of vacuum existing in the U tube is measured by any desiredtype of vacuum gauge. In the present instance, a thermocouple type ofvacuum gauge having a head 71 and an indicator 72 is employed formeasuring high vaccuum. A McLeod type vacuum gauge 73, well known tothose skilled in the art, connected to the port 39, is employed tomeasure pressures greater than 1 mm. of mercury. The corresponding port39a at the other end of the tube is unused and is sealed by a plug. Atemperature indicating and control device 75 is associated with the oven15 in order to maintain the temperature constant at a set value. Suchcontrol devices are also Well known to those skilled in the art.Electric heating of the oven is perferred.

In a practical operating cycle, the resistor elements 45 stacked intrays 46 are inserted into each leg of the nitriding tube. Such stackingis prefer-ably done with care since the alloy film on resistor elementsis somewhat fragile prior to the nitriding operation. The end caps 32,32a are then clamped in place at each end of the U tube.

In carrying out the present invention, the nitriding tube, prior toadmission of ammonia gas at low pressure is filled with ammonia gas atsubstantially atmospheric pressure, thereby to insure removal of all ofthe air from the nitriding chamber. To accomplish this, valve 52 isclosed and the high vacuum is drawn in the nitriding tube 10 throughvalve 56. When the indicated pressure has reached the desired low level,the valve 56 is closed and the valve 52 is opened, along with valve 51,in order to supply a flushing change of ammonia gas to the nitridingtube until the pressure in the tube builds up substantially toatmospheric. Following this, the valve 52 is again closed and the valve56 opened so that the ammonia gas is drawn from the nitriding tube. Thisinsures that any residual gas in the tube is ammonia gas. The valves arethen adjusted so that ammonia gas flows through the nitriding tube at apressure substantially below atmospheric pressure and at a predeterminedrate of flow. The rate of flow is preferably about 1400 cubiccentimeters per minute, although such rate may be anywhere within arange of about 500 to about 5000 cubic centimeters per minute. Underfiow conditions the pressure in the nitriding tube is preferably about 7to 9 millimeters of mercury, although, here again, some variation ispermissible. Thus, the pressure may be anywhere between about 5 to about20 millimeters of mercury without substantial sacrifice in the resultobtained. The flow and pressure adjustment are preferably made with thenitriding tube in the oven and after the nitriding tube has been broughtup to operating temperature. The temperature settable on the controldevice 75 is preferably about 1000 C. although successful operation maybe achieved within the range of about 900 to 1200 C.

After the flow conditions have been maintained for about one half hourthe solenoid reversing timer energizes the solenoid valve 60 to switchthe ports 63, 64 so that ammonia gas flows in end 12 of the U tube,being evacuated from end 11. After one hour the tube 10 is withdrawnfrom the oven. There is little benefit in maintaining the nitriding tubeat elevated temperature for more than one hour. Stability of the filmhas been achieved by subjecting the resistor elements to treatment foras little as one half hour. However, continuing the nitriding conditionsfor a longer period, up to say, two hours, provides a safety margin andinsures that nitriding has been carried to completion.

The nitriding tube is allowed to cool gradually at room temepratureuntil it can be comforatably handled. The flow of ammonia gas is thenstopped and the system is evacuated to /2 mm. of mercury or less inorder to get rid of the residual ammonia gas. The vacuum is thenreleased following which the tube is opened for removal of the resistorelements.

It is found that one of the advantages of employing ammonia gas atpressures substantially below atmospheric, and within the range of about5 to about 20 millimeters of mercury, is that the mean free path of theammonia gas molecules is substantially increased over the value existingat atmospheric pressure. Consequently, the ammonia molecules havegreater energy content and more complete conversion of the film isachieved than is possible using gas at atmospheric pressure. It has beenfound desirable to continue the constant flow of the low pressureammonia gas during the entire nitriding period. Using the describedprocedure, complete nitriding is achieved even over those areas wherethe resistor elements are in contact. It is found that the nature of thenitriding film is improved over that obtained at atmospheric pressure.The films are uniformly hard and adherent with extremely good physicaland chemical stability and minimum temperature coefficient ofresistance.

We claim as our invention:

1. The process of nitriding film type resistor elements having a film ofmetal capable of being nitrided thereon which comprises placing aplurality of resistor elements in a nitriding chamber, flowing ammoniagas through said chamber at a pressure of approximately 5 toapproximately 20 millimeters of mercury and at a rate Within the rangeof about 500 to about 5000 cc. per minute, and maintaining thetemperature in the chamber Within a range of about 900 to about l200C.during the interval that the resistors are maintained at the elevatedtemperature and until the film has been converted to nitride form.

2. The process of nitriding film type resistor elements having a film ofmetal capable of being nitrided thereon which comprises placing aplurality of resistor elements in a nitriding chamber, evacuating theair from said chamber, flowing ammonia gas through the chamber at apressure of approximately 5 to approximately 20 millimeters of mercury,and maintaining the temperature of the chamber Within a range of about900 to about 1200C. during the intereval that the resistor elements aremaintained at the elevated temperature and until the film has beenconverted to nitride form.

3. The process of nitriding film type resistor elements having a film ofmetal capable of being nitrided thereon which comprises placing aplurality of resistor elements 20 in a nitriding chamber, evacuating theair from said chamber, charging the chamber With ammonia gas at apressure of approximately 5 to approximately 20 millimeters of mercury,maintaining the temperature of the chamber within a range of about 900to about 1200 C. until nitriding is complete and gradually replenishingthe gas during the interval that the elements are maintained at theelevated temperature.

References Cited by the Examiner DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

1. THE PROCESS OF NITRIDING FILM TYPE RESISTOR ELEMENTS HAVING A FILM OFMETAL CAPABLE OF BEING NITRIDED THEREON WHICH COMPRISES PLACING APLURALITY OF RESISTOR ELEMENTS IN A NITRIDING CHAMBER, FLOWING AMMONIAGAS THROUGH SAID CHAMBER AT A PRESSURE OF APPROXIMATELY 5 TOAPPROXIMATELY 20 MILLIMETERS OF MERCURY AND AT A RATE WITHIN THE RANGEOF ABOUT 500 TO ABOUT 5000 CC. PER MINUTE, AND MAINTAINING THETEMPERATURE IN THE CHAMBER WITHIN A RANGE OF ABOUT 900 TO ABOUT 1200*C.DURING THE INTERVAL THAT THE RESISTORS ARE MAINTAINED AT THE ELEVATEDTEMPERATURE AND UNTIL THE FILM HAS BEEN CONVERTED TO NITRIDE FORM.